1. Field of the Invention
The present invention relates to a structural design for improving the quality of high frequency signal transmission of a circuit board, and in particular to a structure of a flexible circuit board that includes an attenuation reduction grounding pattern structure and the attenuation reduction grounding pattern structure is provided corresponding to connection pads carrying high frequency signals of the flexible circuit board.
2. The Related Arts
Due to the amount of data transmitted through signal lines being increasingly large, electronic devices of the present day requires an increased number of signal transmission lines and also needs the signals to be transmitted in increasingly higher frequencies. Thus, at present, differential mode signal transmission technology has been widely used to reduce electromagnetic interference (EMI). For example, such a technology has been largely used for signals of USB, LVDS (Lowe Voltage Differential Signaling), and EDP (Embedded Display Port) to reduce EMI.
In the differential mode signal transmission technology, two differential mode signal lines are coupled to form a signal line pair for transmission of signals that have the same amplitude but opposite phases. An external interference signal will induce common-mode noises having identical amplitude and phase in the two signal lines and will be expelled by a differential input pair of an integrated device so as to achieve improved suppression of EMI in the circuit.
Although the differential mode signal transmission can greatly improve potential problems that might occur in the transmission of signals, in a practical application, a poor design might result in problems associated with signal reflection, scattering of electromagnetic signals, loss of signal transmission and receipt, and distortion of waveforms of signals. These problems get severe particularly for flexible circuit boards having reduced substrate thickness. Factors that cause such problems include: poor matching of characteristic impedance of the differential mode signal lines in extension directions, poor control of excessive stray capacitance effect between high frequency connection pads and a grounding layer, non-matching of characteristic impedance of the differential mode signal lines and the high frequency connection pad.
Further, for example, when a flexible circuit board is plugged into an insertion slot of a female socket connector, the differential mode signal lines and the high frequency connection pads will induce parasitic capacitance and inductance with respect to conductive terminals inside the female socket connector, which cause reflection and loss of the high frequency components of digital signals and thus affect the quality of high speed digital signal transmission.
Further, for example, in an application where a connector is mounted on a flexible circuit board, the differential mode signal lines and the high frequency connection pads will induce parasitic capacitance and inductance with respect to signal connection pins of the connector, which also affect the quality of high frequency signal transmission.
In the known techniques, suppressing interference of electromagnetic wave radiation in a length extension direction of the differential mode signal lines and overcoming the problems of impedance matching have been researched and various techniques have been proposed. However, for the connection site, as well as neighboring areas, between the differential mode signal lines and the high frequency connection pads provided on a flexible circuit board, due to the line width of the differential mode signal line (the line width being extremely small) and the dimension specifications of the signal connection pins and components of the connector (which are of relatively large sizes as compared to the line width of the signal lines), up to date, there is still no satisfactory solution proposed by the business to ensure the quality of high frequency signal transmission.
Further, in applications where a flexible circuit board is plugged into an insertion slot of a female socket connector and a connector is provided on a flexible circuit board, there is still no satisfactory solution for handling the issues regarding the quality of high frequency signal transmission between the differential mode signal lines and the high frequency connection pads and the conductive terminals of the female socket connector and the signal connection pins of the connector.